Planar Lightwave Circuits (PLCs) are optical systems comprising one or more waveguides integrated on the surface of a substrate, wherein the waveguides can be combined to provide complex optical functionality. These “surface waveguides” typically include a core of a first material that is surrounded by a second material having a refractive index that is lower than that of the first material. As a result, light propagating through the core of a surface waveguide is guided along the waveguide by the core due to internal reflection at the interface between the core and cladding materials.
Historically, surface waveguides have been based on materials whose refractive indices were only slightly different (<1%). Such “low-contrast waveguides” are typically based on a core comprising doped silicon dioxide, where the doping provides a slight increase in refractive index from an undoped silicon dioxide cladding. Low-contrast waveguides were developed for use in telecommunications systems, where low propagation loss is critical. Low-contrast waveguides can have propagation losses less than 0.1 dB/cm.
Because the refractive index difference between the core and cladding materials is small, light is only loosely confined in the core of a low-contrast waveguide and a significant portion of its optical energy extends well out into the cladding as an evanescent tail. As a result, the mode-field profile of a light signal (i.e., the distribution of optical energy about the central axis of the waveguide) propagating in a low-contrast waveguide is quite large. It is fairly well matched to that of a conventional optical fiber, however. Low-contrast waveguides, therefore, can optically couple light into and out of a conventional optical fiber with very low loss. This high coupling efficiency enables inclusion of low-contrast waveguide-based PLCs in the optical fiber plant that forms the backbone of modern telecommunications and data communications networks.
Unfortunately, the large mode-field profile can lead to light leaking out of the waveguide—particularly at tight bends and loops. As a result, low-contrast waveguides are normally routed along the substrate using large bending radii. Further, to avoid overlap of the mode-field profiles of adjacent waveguides, low-contrast waveguides must be spaced well apart to mitigate optical coupling between them. PLCs based on low-contrast waveguides, therefore, require a great deal of chip real estate to realize any significant functionality. In addition, the large-bending radii requirement of low-contrast waveguides precludes realization of some waveguide components, such as large free-spectral-range ring resonators, which require small bend radii.
High-contrast waveguides, on the other hand, employ core and cladding materials having a large difference in refractive index (typically 25-100%). As a result, a high-contrast waveguide more tightly confines optical energy to inside the core itself, realizing only small evanescent tail in the cladding (i.e., they are characterized by a relatively smaller mode-field profile). Light leakage in a high-contrast waveguide is mitigated and, therefore, high-contrast waveguides can be routed with tight bending radii and can also be spaced more densely than low-contrast waveguides. This enables PLCs that require very little chip real estate, by comparison with low-contrast waveguide-based PLCs.
Unfortunately, high-contrast waveguides typically exhibit relatively high propagation loss. Further, their small mode-field profile is not well matched to that of a conventional optical fiber, which leads to large optical loss when a high-contrast waveguide is optically coupled with a conventional optical fiber.
In order to improve optical coupling efficiency between a high-contrast waveguide and a conventional optical fiber, the waveguide-taper-based spotsize converter has been developed to effect a size change the mode-field profile at a waveguide facet so that the mode-field profile at the facet is more closely matched to that of an optical fiber.
Attempts to form such spotsize converters in the prior art have typically relied on waveguide regions comprising a one-dimensional taper in the lateral dimension, wherein the lateral taper is formed using conventional photolithography and etching. Examples of such devices are described in “Spotsize converters for rib-type silicon photonic wire waveguides,” Proceedings of the 5th International Conference on Group IV Photonics, Sorrento, Italy, September 17-19, pp. 200-202 (2008) and “Low loss shallow-ridge silicon waveguides,” Optics Express, Vol. 18, No. 14, pp. 14474-14479 (2010). Unfortunately, only marginal coupling efficiency improvement is obtained via such spotsize converters.
Of more promise, however, are spotsize converters that are tapered in two dimensions, such as described in “Low-Loss Compact Arrayed Waveguide Grating with Spotsize Converter Fabricated by a Shadow-Mask Etching Technique,” Electronics and Telecommunications Research Institute (ETRI) Journal, Vol. 27, No. 1, pp. 89-94 (2005). While the structure of these spotsize converters shows great promise for low fiber-to-chip coupling losses, shadow-mask etching is extremely difficult to control. As a result, spotsize converters fabricated in this manner are expensive to produce in volume and are likely to suffer from variations in performance as well, making them difficult, at best, to commercialize.
Historically, the drawbacks associated with high-contrast-waveguide-based PLCs has limited their use in telecommunications or data communications applications. As a result, they are primarily used in applications where optical loss of less concern, such as sensor applications.
A low-cost, reproducible surface-waveguide technology that has low optical propagation loss, supports tight bending radii and dense integration, and that can be efficiently optically coupled with external devices, such as optical fibers, lasers, detectors, and the like, would represent a significant advance in the state-of-the-art.